(a) Field of the Invention
The present invention relates to a large relative aperture ratio zoom lens system having a focal length on the order of 35 to 70 mm for use with 35 mm single lens reflex cameras.
(b) Description of the Prior Art
Out of zoom lens systems, a certain type consisting of two lens groups comprises a front lens group having negative refractive power, a rear lens group having positive refractive power and a stop arranged in said rear lens group. This type is suited as a wide-angle zoom lens system having a zooming ratio on the order of 2.times. since it is capable of suppressing to relatively low level the variation of distortion which is known as a defect of the zoom lens systems consisting of four lens groups and permits shortening total length of said zoom lens system. For this reason, researches have recently been made on this type of zoom lens system consisting of two lens groups, making rapid progress in this field and realizing numerous conventional examples.
However, it is difficult to design a zoom lens system so as to have a large relative aperture, and the conventional examples had F numbers of 3.5 at best. For example, the zoom lens systems disclosed by Japanese Unexamined Published Patent Applications No. 2548/74, No. 132360/78 and No. 19022/81 have an F number of 3.5 or a small relative aperture. The small relative aperture of the zoom lens systems comprising two lens groups is traced to their basic composition. Speaking concretely, the front lens group having a strongly negative refractive power remarkably diverges rays, which remarkably enlarges relative aperture of the rear lens group at the stage of incidence on said lens group. As a result, spherical aberration is remarkably undercorrected at the tele position. In a case where the rear lens group is designed as a triplet type, the image side surface of the negative lens element must have a strong power for the purpose of correcting said undercorrected spherical aberration. This power results in curvature of astigmatism characteristics and uneven image plane at the wide position of the zoom lens system. Further, it aggravates coma and degrades contrast, thereby lowering image quality.